An Open Auction is a certain type of bidding process in programmatic advertising.
Within an open auction the publisher’s advertising inventory is offered in a bidding process which is open to every advertiser who wants to make a bid for the inventory. Thus it is the opposite of an Invitation-Only-Auction which only allows selected advertisers to participate in the real-time bidding process.
In the true sense of an open auction, the buyer is responsible for paying for the ad space they outbid everyone else. By bidding more than the others, the winning bidder is agreeing to accept responsibility for paying for the offered inventory.
Open auctions are basically designed to ensure that the publisher is earning the maximum possible revenue for his ad inventory. The more advertisers appear to bid for an ad space, the higher the competition and the more publishers can earn selling their inventory.